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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
AFS Fixed Income Securities		$ 5,945,360	$ 6,095,620
Equity Securities		99,989	72,937
Short-term investments		517,558	282,490
Net Asset Value		39,200	37,800
Fair Value, Recurring [Member]
AFS Fixed Income Securities		5,945,360	6,095,620
Equity Securities		99,989	72,937
Short-term investments		517,558	282,490
Total assets measured at fair value		6,562,907	6,451,047
U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		142,237	116,186
Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		12,886	18,542
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,197,213	1,230,090
Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,909,504	1,947,477
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		722,261	793,012
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,430,226	1,451,969
Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		531,033	538,344
Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	98,410	69,900
Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		1,579	3,037
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		44,093	44,718
Equity Securities		60,754	35,182
Short-term investments		504,243	265,306
Total assets measured at fair value		609,090	345,206
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		44,093	41,083
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	3,635
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	59,175	32,145
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		1,579	3,037
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		5,837,820	6,016,817
Equity Securities		0	0
Short-term investments		13,315	17,184
Total assets measured at fair value		5,851,135	6,034,001
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		98,144	75,103
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		12,886	18,542
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,194,323	1,230,090
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,885,319	1,930,426
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		685,889	772,343
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,430,226	1,451,969
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		531,033	538,344
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	0	0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		63,447	34,085
Equity Securities		0	0
Short-term investments		0	0
Total assets measured at fair value		63,447	34,085
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		2,890	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		24,185	17,051
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		36,372	17,034
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	0	0
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		$ 0	$ 0
[1] Investments amounting to $39.2 million at March 31, 2020, and $37.8 million at December 31, 2019, were measured at fair value using net asset value per share (or its practical expedient) and are not classified in the fair value hierarchy. These investments are not redeemable and the timing of liquidations of the underlying assets is unknown at each reporting period disclosed above. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.

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